JPH0794691B2 - Method for producing Cr-Ni series stainless steel thin plate with excellent surface quality and material - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial field of application] The present invention casts a slab having a thickness close to the product thickness by a so-called synchronous continuous casting method in which a mold moves in synchronization with the slab, The present invention relates to a method for producing a Cr-Ni-based stainless steel sheet by series cold rolling without hot rolling.

[Conventional technology]

Conventionally, in order to produce a stainless steel thin plate by the continuous casting method, the mold is vibrated in the casting direction, cast into a slab with a thickness of up to about 100 mm, and the slab obtained is surface-cleaned and then heated. After heating to a temperature of 1000 ° C or higher in a furnace and hot rolling with a hot strip mill consisting of a rough rolling mill and a finish rolling mill, hot strips with a thickness of about several mm are obtained, and after cold rolling , Required annealing,
It was pickled and temper-rolled to obtain a cold-rolled product.

To ensure the required shape (flatness), material (grain size, mechanical properties), and surface quality (prevention of roping) of the final product before cold rolling the hot strip obtained by hot rolling In addition, hot-pressing plate annealing was performed to soften the hot strip in a work-hardened state that was subjected to strong hot working, and pickling and grinding were performed to remove oxide scale and the like on the surface.

Since such a conventional process requires a large amount of energy for heating and processing the material of a long hot rolling facility, it cannot be said to be an excellent manufacturing process from the viewpoint of productivity.

In addition, since the finally obtained thin plate product has a strong residual texture that has developed during hot working, the anisotropy caused by this texture should be taken into consideration when the user presses. There were many restrictions in using the product, such as having to do so.

Therefore, a long hot rolling facility for hot rolling a slab with a thickness of 100 mm or more to form a hot strip and a large amount of energy are not required, and the product can be used due to the texture of hot working. In order to eliminate the above restriction, the development of a process in which the continuous casting process is directly connected to the cold rolling process without passing through the hot rolling process is underway. That is, it is a process of continuously casting a slab (thin band) having a thickness equal to or close to that of a hot strip conventionally obtained by hot rolling, and cold rolling the thin slab. Such a process is described, for example, in "Paper and Steel," Vol. 85, A197-A256, and as a method of continuous casting, the thickness of the slab to be obtained is 1 to 10 mm. The twin-drum method is being considered for the case of about 50 to 50 mm, and the twin-belt method is being considered for the case where the thickness of the cast piece is about 20 to 50 mm.

[Problems to be Solved by the Invention]

However, even if any of the above continuous casting methods is used, the thin plate products produced by the continuous casting / cold rolling process (hereinafter, referred to as "strip continuous casting") without hot rolling are / Hot rolling / cold rolling process (hereinafter abbreviated as "conventional method") has a fine-grained structure and a lower elongation than thin plate products, which reduces the workability of users such as pressing. There was a problem. This phenomenon has been reported, for example, in the paper featured in "CAMP ISIJ vol.1,1988,1670-1705.
By annealing the slab, concretely 1 to 16 of p1697
In the 4th row, "Comparing the solution treated at 1150 ° C x 1 min. After casting and the homogenized at 1150 ° C x 3h,
In both cases, the latter of the materials has increased its growth by several percent. It is described that the δ-ferrite remaining in the cast piece disappears by such a time-order homogenization treatment.

As a result of a detailed study of the Cr-Ni-based stainless steel sheet manufacturing process by the present inventor by strip continuous casting, the growth of recrystallized grains during cold rolling / annealing due to the presence of δ ferrite and inclusions remaining in the cast piece It was suppressed, and it became clear that this was the cause of the fine grain structure of the final product and the decrease in elongation thereof. Therefore, in order to eliminate the decrease in elongation of the product manufactured by continuous strip casting, it is necessary to adjust the components so as to reduce the amount of inclusions and deoxidize, and to perform heat treatment to eliminate δ ferrite.

δ-ferrite can be eliminated by annealing the slab. However, the δ ferrite remaining in the slab does not sufficiently transform into the γ phase in the short-time annealing that is performed for the conventional austenitic stainless steel hot-rolled steel strip, and the above-mentioned 1150 ° C × 3h It is necessary to anneal at high temperature for a long time like above, which is extremely disadvantageous from the viewpoint of productivity and manufacturing cost.Therefore, a more economical heat treatment method or a method of increasing the disappearance rate of δ ferrite during heat treatment is necessary. Met.

The present invention, there is no relative speed difference between the slab and the mold wall, by cold rolling without going through hot rolling by casting a slab of a size close to the product thickness by the so-called synchronous continuous casting method. Cr-Ni
In the method of manufacturing a series of stainless steel sheets, the residual δ ferrite of the cast slab is transformed into the γ phase without the need for annealing at high temperature for a long time, thereby promoting the growth of recrystallized grains during cold rolling annealing. An object of the present invention is to provide a method for producing a soft Cr-Ni-based stainless steel thin plate having excellent properties.

[Means for Solving the Problems]

According to the present invention, the above object is to achieve a thickness of 6 mm or less from a Cr-Ni system stainless steel represented by 18% Cr-8% Ni steel by a continuous casting method in which a mold moves in synchronization with a slab. In the method of casting a slab of, and directly performing cold rolling without hot rolling to produce a thin plate product, the slab immediately after casting is heated to 800 ° C.
After winding at a temperature of 1200 ° C or higher, annealing at 1000 ° C or higher for a few minutes or less, followed by pickling, and cold rolling, and final annealing or final annealing by bright annealing after pickling And then temper rolling to obtain a thin plate product, which is achieved by a method for producing a Cr-Ni-based stainless steel thin plate having excellent surface quality and material.

The present inventor casts by increasing the solid fraction of the slab when leaving the mold, and winding the slab at high temperature immediately after casting, and utilizing the annealing effect of the heat retained by the obtained coil itself. In this case, the residual δ ferrite in the slab can be sufficiently transformed into the γ phase and eliminated without annealing separately at high temperature for a long time, and further, the solid fraction of the slab when leaving the mold can be improved. It was found that the δ-ferrite disappeared earlier by raising it to a higher temperature.

Figure 1 shows a JIS SUS304 stainless steel slab (thickness 2 mm) cast by twin drum continuous casting.
A representative example of the relationship between the winding temperature immediately after casting and the roping height of the final cold rolled product and the elongation in the tensile test is shown. As is clear from the figure, it is necessary to set the coiling temperature to 800 ° C. or higher in order to sufficiently eliminate δ ferrite and obtain a practically sufficient elongation value (48% or higher).
However, in order to suppress the growth of γ grains during winding and maintain the roping height at a value that does not pose a practical problem (0.2 μm or less), the winding temperature needs to be 1200 ° C. or less.
The thin plate products manufactured from the slabs wound in this temperature range are
It was confirmed that the material had practically sufficient material (elongation) and surface quality (prevention of roping).

Furthermore, as the properties of δ ferrite remaining in the cast slab,
It is necessary that the δ ferrite has an unstable composition such that it easily transforms into the γ phase in the winding temperature range. For that purpose, it is necessary to perform rapid solidification that can suppress the concentration of ferrite stabilizing elements (Cr, Si, Mo, Ti, etc.) in δ ferrite, and for that purpose, the slab is removed from the mold wall. It is important to set the solid phase ratio to 65% or more when leaving the mold, and to reduce the equiaxed crystal part (the part where the solidification rate is slower than the columnar crystal part and the stabilization of δ ferrite is advanced) after leaving the mold. .

From this point of view, the present inventor casts a thin cast piece under various casting conditions, and as a result of examining various factors affecting the disappearance rate of δ ferrite, the solidification structure of the cast piece is large in the disappearance rate of δ ferrite. It was found to affect. That is, it was revealed that, under the same slab annealing condition, the disappearance rate of δ ferrite in the columnar crystal part, which is the rapidly solidified structure of the slab, is considerably higher than that in the equiaxed crystal part.

FIG. 2 shows that for JIS SUS304 stainless steel, the ratio of columnar crystals to the solidification structure of the cast pieces (columnar crystal ratio) was changed by changing the casting conditions variously, and the obtained cast pieces were subjected to the same conditions ( A representative example of the results of measuring the elongation of a thin plate product manufactured from the product annealed at 800 ° C x 60 minutes) is shown. As is clear from the figure, it becomes clear that the elongation of the thin plate product improves as the columnar crystal ratio of the slab increases, and particularly when the columnar crystal ratio becomes 65% or more, a remarkably excellent elongation value can be obtained. It was That is, the slab is wound at a high temperature to eliminate the δ ferrite, and the solid fraction is set to 65% or more in order to increase the disappearance rate of the δ ferrite so that the δ ferrite disappears in a short time even at lower temperature annealing. More desirable.

Hereinafter, the present invention will be described in more detail with reference to Examples.

〔Example〕

Various austenitic stainless steels having a basic composition of 18% Cr-8% Ni stainless steel shown in Table 1 are melted,
It was cast into a 2 mm-thick slab by an internal water-cooled twin-drum type continuous casting machine. The solid phase ratio (columnar crystal ratio) when the slab was separated from the drum was suppressed to 80 to 60% by adjusting the drum gap.

The obtained slab is annealed at 1100 ° C x 3 sec., Pickled and then 50%
After cold rolling and annealing, temper rolling with an elongation of 1% was performed to obtain a thin plate product.

[Comparative example]

For comparison, a thin plate product was manufactured by the same procedure from a cast piece which had a solid fraction of 60% and was wound at 700 ° C or 400 ° C.

Table 2 shows the results of evaluating the grain size (GSN), elongation and surface quality of the thin plate products of the examples (Samples A, B and C) and the comparative examples (Samples D and E).

The thin plate product produced by the method of the present invention has a product grain size G.
The SN was 8.0% or less, the elongation was 50% or more, sufficiently satisfying the practically required value (48% or more), and the surface quality was the roping height of 0.2 μm or less, which was the practically sufficient value. .

On the other hand, the comparative example which does not satisfy the winding temperature of the present invention has a fine grain structure with product grain size GSN of 10.5 (D) and 9.6 (E), and the surface quality is good corresponding to this grain size. However, the elongations were 43% (D) and 45% (E), and practically sufficient values were not obtained.

[Effects of the Invention] As described above, according to the present invention, in a manufacturing method by continuous casting in which a cold rolling process is directly performed without performing hot rolling from a casting process, high temperature and long time annealing are not required. Without the residual δ ferrite in the cast slab being sufficiently eliminated,
Cr with excellent workability while ensuring practically sufficient surface quality
-Ni-based stainless steel thin plate can be manufactured. As a result, compared to the conventional method in which a thick cast piece up to about 100 mm was hot-rolled, the manufacturing cost was significantly reduced, and a highly productive Cr-Ni-based stainless steel thin plate manufacturing method was realized. It

[Brief description of drawings]

FIG. 1 is a graph showing the relationship between the winding temperature of the slab and the roping height and elongation of the thin plate product, and FIG. 2 is a typical example of the relationship between the columnar crystal ratio of the slab and the elongation of the product sheet. It is a graph which shows.

Claims (2)

[Claims] 1. A slab having a thickness of 6 mm or less is cast from a Cr-Ni type stainless steel represented by 18% Cr-8% Ni steel by a continuous casting method in which a mold moves in synchronization with the slab. In a method for directly producing cold rolled sheets without hot rolling, a slab immediately after casting is wound at a temperature of 800 ° C or higher and 1200 ° C or lower, and then annealed, pickled, and cooled. Cr-Ni type stainless steel with excellent surface quality and material characterized by being subjected to final rolling by hot rolling and then pickling and then final annealing by bright annealing and then temper rolling to obtain a thin plate product. Manufacturing method of steel sheet. 2. The solid fraction of the slab when separated from the mold wall is 65%.
The method according to claim 1, wherein casting is performed as described above.